5. Summary of Data Reported and Evaluation

5.1 Exposure data

Exposure of high levels of arsenic in drinking-water has been recognized
for many decades in some regions of the world, notably in China, Taiwan (China)
and some countries in Central and South America. More recently, it has been
discovered that a number of other regions have drinking-water that is highly
contaminated with arsenic. In most of these regions, the drinking-water source
is groundwater, naturally contaminated from arsenic-rich geological formations.
The primary regions where high concentrations of arsenic have been measured
in drinking-water include large areas of Bangladesh, China and West Bengal
(India) and smaller areas of Argentina, Australia, Chile, Mexico, Taiwan
(China), the USA and Viet Nam. In some areas of Japan, Mexico, Thailand and
other countries, mining, smelting and other industrial activities have contributed
to elevated concentrations of arsenic in local water sources.

Levels of arsenic in affected areas may range from tens to hundreds or
even thousands of micrograms per litre, whereas in unaffected areas levels
are typically only a few micrograms per litre. The WHO guideline recommends
that levels of arsenic in drinking-water should not exceed 10 micrograms
per litre. Arsenic occurs in drinking-water primarily as arsenate (AsV), although in reducing environments significant
concentrations of arsenite (AsIII) have also been reported. Trace amounts
of methylated arsenic species are typically found in drinking-water, and
higher levels are found in biological systems. Inorganic arsenic (arsenate
plus arsenite) is the predominant form of arsenic in drinking-water.

In many areas where contamination of drinking-water by arsenic has been
reported, current exposures have been reduced by various interventions.

5.2 Human carcinogenicity
data

In previous monographs, the evidence of carcinogenicity to humans of
exposure to arsenic and arsenic compounds, such as medical treatment with
Fowler’s solution and inhalation exposure of mining and smelting workers,
was evaluated as sufficient.

Informative epidemiological studies of cancer in relation
to arsenic in drinking-water include ecological studies and fewer case–control
and cohort studies. For most other known human carcinogens, the major source
of causal evidence derives from case–control and cohort studies, with little
evidence from ecological studies. In contrast, for arsenic in drinking-water,
ecological studies provide important information for causal inference.
The reasons include large exposure contrasts and limited population migration.
As a consequence of widespread exposure to local or regional water sources,
ecological measurements provide a strong indication of individual exposure.
Moreover, in the case of arsenic, the ecological estimates of relative
risk are often so high that potential confounding with known causal factors
cannot explain the results. Hence, in the reviews that follow, ecological
studies are presented in detail.

Urinary bladder cancer

The Working Group evaluated ecological studies in Taiwan (China), Chile,
Argentina and Australia, cohort studies from Taiwan, Japan and the USA and
case–control studies in Taiwan, the USA and Finland.

There is extensive evidence of increased risks for urinary bladder cancer
associated with arsenic in drinking-water. All studies that involved populations
with high long-term exposures found substantial increases in the risk for
bladder cancer. Key evidence derives from ecological studies in Taiwan and
Chile. In Taiwan, the evidence is supported by case–control studies and cohort
studies within the exposed communities that demonstrate evidence of dose–response
relationships with levels of arsenic in drinking-water. The evidence of increased
mortality from bladder cancer in Chile comes from a large population with
exposure to arsenic in all major cities and towns of the contaminated region.

There is also evidence of increased risks for bladder cancer from a small
cohort study in Japan of persons drinking from wells that had been highly
contaminated with arsenic wastes from a factory and an ecological study from
Argentina with moderate exposure to arsenic in well-water. Two case–control
studies that investigate low exposure to arsenic found increased risks with
increasing exposure in one or more subgroups.

Considered overall, the findings cannot be attributed to chance or confounding,
and they are consistent, with strong associations found in populations with
high exposure. There is evidence of dose–response relationships within exposed
populations.

Lung cancer

The Working Group evaluated ecological studies using mortality data in
Taiwan (China), Chile, Argentina and Australia, cohort studies in Taiwan,
Japan and the USA and case–control studies in Taiwan and Chile.

Increased risk for lung cancer was consistently observed in ecological,
case–control and cohort studies in Taiwan, Japan, Chile and Argentina. Evidence
for a dose–response relationship between arsenic in drinking-water and risk
for lung cancer was also observed in ecological studies in Taiwan and Argentina,
in cohort studies in south-western and north-eastern Taiwan and Japan and
in case–control studies in south-western Taiwan and Chile. The potential
confounding effect of cigarette smoking was ruled out by direct and indirect
evidence in studies from Taiwan and Chile.

Considered overall, the findings cannot be attributed to chance or confounding,
are consistent and demonstrate strong associations in populations with high
exposure. There is evidence of a dose–response relationship.

Skin cancer

The Working Group evaluated ecological studies from Taiwan (China), Mexico,
Chile and the USA, cohort studies from Taiwan and a case–control study from
the USA.

The recognition that arsenic was potentially carcinogenic arose from
occurrences of skin cancer after ingestion of medicinal arsenic, arsenical
pesticide residues and arsenic-contaminated drinking-water. Skin cancer is
a commonly observed malignancy related to contamination of drinking-water
with arsenic. The characteristic arsenic-associated skin tumours include
keratinocytic malignancies (non-melanoma skin cancers), in particular squamous-cell
carcinomas, including Bowen disease, and multiple basal-cell carcinomas.

Ecological studies, largely from the south-west of Taiwan, indicate substantially
elevated incidence of, prevalence of and mortality rates for skin cancer
associated with drinking-water highly contaminated with arsenic, with evidence
of a dose–response relationship. Findings in ecological studies were substantiated
in two cohort studies in the region of Taiwan that is endemic for arsenic.
Increased mortality from skin cancer was found in Chile. A high prevalence
of skin lesions, including skin cancers, was found in rural regions of Mexico.
An excess risk for skin cancer was observed in a case–control study in the
USA conducted in an area with lower concentrations of arsenic in drinking-water.
A cohort study from the south-west of Taiwan reported that differences in
the levels of serum beta-carotene and urinary arsenic metabolites may modify
the risk for arsenic-induced skin cancers.

Liver cancer

The Working Group evaluated ecological studies using mortality data in
Taiwan (China), Chile, Argentina and Australia, cohort studies in Taiwan,
Japan and the USA and a case–control study in Taiwan of liver cancer cases
identified from death certificates.

Increased mortality from liver cancer was observed in the ecological
studies involving a large population with high exposure to arsenic in Taiwan.
Evidence for a dose–response relationship between arsenic in drinking-water
and liver cancer mortality was observed in both ecological and case–control
studies in Taiwan. Increased risks were also found in small cohort studies
in Taiwan and Japan. Findings on mortality from liver cancer observed in
ecological studies in Chile are inconsistent.

The interpretation of these findings is limited by the small number of
liver cancer cases, questionable accuracy of the diagnosis of liver cancer
on death certificates and potential confounding or modifying effects of chronic
hepatitis virus infection or other factors.

Kidney cancer

The Working Group evaluated ecological studies in Taiwan
(China), Chile, Argentina and Australia, and cohort studies from Taiwan
and the USA.

All studies that involved populations with high long-term exposures to
arsenic found increased risks for kidney cancer. Key evidence comes from
ecological studies in Taiwan and Chile. In Taiwan, the evidence is supplemented
by a small cohort study of patients with Blackfoot disease. The evidence
of increased mortality from kidney cancer in Chile comes from a large population
with exposure to arsenic in all major cities and towns of the region. There
is also evidence of increased risk for kidney cancer in populations in Argentina
with moderate exposure to arsenic in well-water.

Relative risk estimates for kidney cancer were generally lower than those
for urinary bladder cancer, and no studies have reported dose–response relationships
on the basis of individual exposure assessment.

Other cancers

The Working Group evaluated ecological studies from Taiwan (China), Chile
and the USA, cohort studies from Japan and the USA and one case–control study
each from Canada and the USA.

Excess mortality from prostate cancer was found in south-west Taiwan.
Inconsistent findings were reported for other cancer sites.

5.3 Animal carcinogenicity
data

Dimethylarsinic acid was tested for carcinogenicity by administration
in drinking-water in mice and rats. It was also tested in two-stage initiation–promotion
studies in mice and rats. Complete carcinogenicity was observed in the urinary
bladder of rats and lungs of mice. Dimethylarsinic acid exerted its carcinogenic
effect on spontaneous development of tumours in p53+/– and p53+/+ mice.
Dimethylarsinic acid is a tumour promoter in the skin and lung of mice, and
in the liver, urinary bladder, kidney and thyroid gland of rats.

Arsenic in drinking-water is well absorbed in the gastrointestinal tract.
The trivalent species of arsenic are formed in vivo after exposure
to pentavalent arsenic. Arsenic is metabolized by a series of reductions
and oxidations and by methylation reactions. Methylated trivalent arsenic
is more toxic and less genotoxi than trivalent inorganic arsenic; in contrast,
methylated pentavalent arsenic is less toxic and less genotoxic than pentavalent
inorganic arsenic. There is a large variation in metabolism between animal
species, population groups and individuals. Both inorganic arsenic and its
methylated metabolites are excreted in urine.

Acute effects due to ingestion of arsenic are characterized by severe
vomiting and diarrhoea with features of shock, muscle cramps and cardiac
abnormalities. Subacute exposures affect primarily the respiratory, gastrointestinal,
cardiovascular, nervous and haematopoietic systems.

Most reports of chronic arsenic toxicity focus on skin
manifestations such as pigmentation, with depigmentation affecting trunks
and limbs and keratosis affecting hands and feet. Chronic lung disease,
peripheral neuropathy, hepatomegaly and peripheral vascular disease have
frequently been reported in cases of chronic exposure to arsenic. Exposure
to arsenic has been associated with an increased risk for diabetes mellitus.
Other systemic manifestations include cardiovascular effects, abdominal
pain, anorexia, nausea, diarrhoea, cerebrovascular disease, non-pitting
oedema of hands, feet or legs, anaemia and generalized weakness. In a study
in Taiwan (China), significantly higher mortality from cardiovascular and
peripheral vascular disease was reported among patients with Blackfoot
disease compared with the general population of Taiwan or unaffected residents
in endemic areas of Blackfoot disease.

The acute toxicity of trivalent arsenic is greater than that of the pentavalent
form. The 50% lethal dose of arsenic trioxide in mice by the oral route varies
from 15 to 48 mg/kg bw, whereas the acute lethal dose in humans varies from
1 to 3 mg/kg bw. In chronic toxicity studies, arsenic inhibits mitochondrial
respiration and induces apoptosis accompanied by a loss of the mitochondrial
transmembrane potential. Metallothionein is thought to have a protective
effect against the toxicity of arsenic.

Arsenic can modify the urinary excretion of porphyrins in animals and
humans. It also interferes with the activities of several enzymes of the
haeme biosynthetic pathway. The major abnormalities in urinary porphyrin
excretion in chronically exposed humans are (a) significant reductions
in coproporphyrin III excretion, resulting in a decrease in the ratio of
coproporphyrinogen oxidase III to coproporphyrinogen oxidase I and (b)
significant increases in uroporphyrin excretion.

Exposure to arsenite or arsenate results in generation of reduced oxygen
species in laboratory animals and human cells. Exposure to arsenicals either in
vivo or in vitro in a variety of model systems causes induction
of a number of major stress-protein families such as heat-shock proteins.
Recent studies in animals demonstrated altered gene expression following
acute treatment with arsenic that included DNA repair genes, activation of
transcription factors, such as activator protein 1, and an increase in pro-inflammatory
cytokines. All of these events could play a role in the toxicity of arsenic.

Few studies have been conducted on the immunotoxicity of arsenic. All
arsenic compounds evaluated in mouse spleen cells suppressed plaque-forming
cell responses to sheep erythrocytes and proliferative response to mitogens.
Furthermore, arsenic impaired stimulation and proliferation of human lymphocytes in
vitro. Recent studies suggest that apoptosis may be an important mechanism
for arsenic-induced immunosuppression.

Experimental animal studies have demonstrated the developmental toxicity
of trivalent and pentavalent arsenic, monomethylarsonic acid and dimethylarsinic
acid. Limited human data suggest that exposure to high concentrations of
arsenic in drinking-water during pregnancy may increase fetal and neonatal
mortality.

The genotoxicity of arsenic is due largely to the trivalent arsenicals.
In humans, arsenic is a chromosomal mutagen (an agent that induces mutations
involving more than one gene, typically large deletions or rearrangements).
Arsenic appears to have limited ability to induce point mutations. Elevated
frequencies of micronuclei, chromosomal aberrations and aneuploidy were detected
in the peripheral lymphocytes or urothelial cells, or both, of people exposed
to elevated levels of arsenic. In vitro, arsenic was not a point mutagen
in bacteria. In mammalian cells, arsenic caused various types of chromosomal
mutations and aneuploidy. In combination with many genotoxic agents, including
ultraviolet light, arsenic was a synergistic co-mutagen. In vitro,
arsenite was genotoxic at micromolar concentrations. Arsenate was approximately
one order of magnitude less genotoxic than arsenite, dimethylarsinic acid
and monomethylarsonic acid induced genotoxicity at millimolar concentrations.

Methylarsenous acid and dimethylarsinous acid are intermediary metabolites
in the methylation of arsenic. Their genotoxicity has not been fully established,
but recent results implicate a major role for these metabolites and reduced
(reactive) oxygen species in the induction of urinary bladder cancer in rats.

5.5 Evaluation

There is sufficient evidence in humans that arsenic in drinking-water
causes cancers of the urinary bladder, lung and skin.

There is sufficient evidence in experimental animals for the carcinogenicity
of dimethylarsinic acid.

There is limited evidence in experimental animals for the carcinogenicity
of sodium arsenite, calcium arsenate and arsenic trioxide.

There is inadequate evidence in experimental animals for the carcinogenicity
of sodium arsenate and arsenic trisulfide.

Taken together, the studies on inorganic arsenic provide limited evidence for
carcinogenicity in experimental animals.